Due to their small size, high Q values, and very low insertion losses at microwave frequencies, particularly those above 1.5 Gigahertz (GHz), Bulk Acoustic Wave (BAW) devices have been widely used in modern wireless applications. For instance, BAW filters are the filter of choice for many 3rd Generation (3G) and 4th Generation (4G) wireless devices, and are destined to dominate filter applications for 5th Generation (5G) wireless devices. The low insertion loss of the BAW filter provides many advantages such as improved battery life, compensation for higher losses associated with the need to support many frequency bands in a single mobile device, etc.
One example of a conventional Wafer-Level-Packaged (WLP) BAW device 10 is illustrated in FIG. 1. The BAW device 10 includes a BAW resonator 12 that has a substrate 14, a reflector 16 over the substrate 14, and a transducer 18 over the reflector 16. The reflector 16 is typically formed by a stack of reflector layers (not shown), which alternate in material composition to produce a significant reflection coefficient at the junction of adjacent reflector layers. The transducer 18 includes a piezoelectric layer 20, which is sandwiched between a top electrode 22 and a bottom electrode 24. The transducer 18 may also include a bottom electrode lead 26, which resides underneath the piezoelectric layer 20 and is connected to the bottom electrode 24.
In order to protect BAW resonators from external elements (such as moisture, contamination, etc.), housings, also known as WLP walls and caps, are applied to enclose the BAW resonators. In this example, the BAW device 10 includes a WLP enclosure 28, which is coupled to the piezoelectric layer 20 to encapsulate the top electrode 22 of the BAW resonator 12. In addition, the piezoelectric layer 20 has an opening outside the WLP enclosure 28, where a portion of the bottom electrode lead 26 is exposed through the opening of the piezoelectric layer 20. A copper pillar connection 30 with a tin cap 32 is coupled to the exposed portion of the bottom electrode lead 26 to provide an external electrical connection for the BAW device 10.
Because of the processing constraints, there are space requirements between the WLP enclosure 28 and the copper pillar connection 30. In addition, the round shape of the copper pillar connection 30 will also take up a significant area of device space. As such, the opening of the piezoelectric layer 20 needs to be relatively far away from the WLP enclosure 28 to accommodate the copper pillar connection 30. Accordingly, there remains a need for improved BAW device designs to reduce the device size without affecting the external electrical connection of the BAW device. Further, there is also a need to keep the final product cost effective.